1. Field of the Invention
The present invention relates to an optical scanning apparatus used with an image forming apparatus or the like.
2. Description of the Related Art
An optical scanning apparatus used with an image forming apparatus or the like is provided with a light source that emits an optical beam for forming an electrostatic latent image on a photosensitive member. The accuracy of an image forming position of the optical beam and the accuracy of a spot diameter (beam waist position) of the optical beam on the photosensitive member affect the performance of the image forming apparatus. When the image forming position of the optical beam on the photosensitive member is not a desired position, this leads to color shift in the case of a color image forming apparatus, and when the spot diameter changes from an ideal state, fine-line reproducibility drops. Such changes in the image forming position and spot diameter of the optical beam tend to become evident as a result of employing plastic optical members. This is because the shape or refractive index of the plastic optical members changes due to environmental changes in temperature, humidity and the like.
Change in the spot diameter is so-called focus shift. In view of this, in Japanese Patent Laid-Open No. 2006-258838, an optical scanning apparatus that employs an autofocus (AF) mechanism is proposed. According to this optical scanning apparatus, changes in focus and irradiation position are observed using a photodiode module having a V-shaped aperture slit installed in a front surface thereof.
Incidentally, phenomena such as the following arise with an optical scanning apparatus used in an image forming apparatus:
Temperature inside the image forming apparatus rises differently depending on different print modes such as intermittent printing and continuous printing;
Temperature inside the image forming apparatus rises differently depending on the surrounding environment, such as in a situation where the image forming apparatus is suddenly started up when the ambient temperature around the image forming apparatus is low, or where the temperature around the image forming apparatus is suddenly cooled from a high ambient temperature.
When the way in which the spot deteriorates thus changes depending on the change in temperature rise distribution within the image forming apparatus, it is difficult with the invention described in Japanese Patent Laid-Open No. 2006-258838 to accurately measure irradiation position shift and focus shift.